Tantalum is currently used extensively in the electronics industry which employs tantalum in the manufacture of highly effective electronic capacitors. This is mainly attributed to the strong and stable dielectric properties of the oxide film on the anodized metal. Both wrought thin foils and powders are used to manufacture bulk capacitors. In addition, thin film capacitors for microcircuit applications are formed by anodization of tantalum films, which are normally produced by sputtering. Tantalum is also sputtered in an Ar--N.sub.2 ambient to form an ultra thin TaN layer which is used as a diffusion barrier between a Cu layer and a silicon substrate in new generation chips to ensure that the cross section of the interconnects can make use of the high conductivity properties of Cu. It is reported that the microstructure and stoichiometry of the TaN film are, unlike TiN, relatively insensitive to the deposition conditions. Therefore, TaN is considered a much better diffusion barrier than TiN for chip manufacture using copper as metallization material. For these thin film applications in the microelectronics industry, high purity tantalum sputtering targets are needed.
Most of the tantalum metal produced in the world today is derived from sodium reduction of potassium heptafluotantalate (K.sub.2 TaF.sub.7). Processes which are not adapted commercially to any significant extent include the reduction of tantalum oxide (Ta.sub.2 O.sub.5)with metallic reductants such as calcium and aluminum, and non metallic reductants carbon and carbon nitrogen; the reduction of the tantalum pentachloride (TaCl.sub.5) with magnesium, sodium or hydrogen; and the thermal dissociation of TaCl.sub.5.
Reduced tantalum is obtained either as powder, sponge or massive metal. It invariably contains significant amounts of oxygen, as well as other impurities such as reductants and impurities that may be present in the starting tantalum compounds. For removal of impurities in tantalum, electron beam melting is often conducted. During electron beam melting, most of the metallic impurities and interstitial gases are vaporized because of their high vapor pressure at the melting point of tantalum (2996.degree. C.). Essentially all elements, except niobium, tungsten, molybdenum, uranium and thorium can be eliminated this way. While the metallic impurities and nitrogen are removed by direct volatilization, the removal of oxygen takes place via mechanisms involving formation and evaporation of carbon oxides, aluminum oxides, water, as well as suboxides of tantalum. The purity can be further improved by repeated electron beam melting. Other refining processes include vacuum arc melting, vacuum sintering, molten salt electrorefining and tantalum iodide refining, with the iodide process being the most promising technique for removing tungsten and molybdenum.
The above mentioned refining methods are not effective for removal of niobium from tantalum. Since tantalum and niobium are closely associated with each other in nature, the removal of niobium is critical to prepare very high pure tantalum. In practice, their separation is conducted before reduction via methods such as solvent extraction, chlorination and fractional crystallization.
The tantalum target manufacturing process includes forging ingot into billet, surface machining billet, cutting billet into pieces, cold rolling the pieces into blanks, annealing blanks, final finishing and bonding to backing plates.